(1) Field of the Invention
The present invention relates to a method of removing any short-circuit portion in a photoelectric conversion device having a surface layer made of a transparent electrically conductive material.
(2) Related Background Art
Warming of the earth is becoming a matter of great concern as a result of the current increase in carbon dioxide content in the atmospheric air, and this has given a rise to the demand for cleaner energy sources. Nuclear power generation, which still involves the unsolved problem concerning difficulty in disposal of radioactive wastes, is also being demanded to be substituted by cleaner energy sources.
Various clean energy sources are expected to be used in the future, among which solar cells are considered to be promising because of the high degree of cleanness and safety, as well as ease of use. Specifically, studies are being made on amorphous silicon solar cells which may easily be formed as a structure having a large area and which can be produced at low cost. A structure having a large area, however, is impractical as it inevitably suffers from local electric short-circuit, resulting in a lowered product yield.
Japanese Patent Publication No. 62-53958 proposes a method of removing local short-circuit in a photoelectric conversion device such as a solar battery device by applying an energy beam, e.g., a laser beam to the local portion where the short-circuit is taking place. This method, however, is disadvantageous in that it essentially necessitates an operation for locating or determining the portion where the short-circuit is taking place. When short-circuits have taken place in a plurality of portions, laborious work is required in irradiating all these portions with the energy beam.
The specification of U.S. Pat. No. 4,451,970 and Japanese Patent Laid-Open No. 59-94473 proposes a method and a system for removing any short-circuit path in a solar battery device having a semiconductor layer and a conductive light-transmitting layer which are formed on a substrate. According to the proposal, a film is formed from an electrolyte on the conductive light-transmitting layer in the short-circuit area of the device and a voltage is applied such as to maintain a positive potential of the electrolyte film relative to the substrate. These references also show an accumulation of an insulating layer in the area from which the short-circuit current path has been removed. Unfortunately, however, it is impossible to provide the whole electrolyte film with positive charges because the electrolyte film is contacted by both the cathode and anode.
Specifications of U.S. Pat. Nos. 4,510,674 and 4,510,675 propose methods of removing any short-circuit path defect. After determining the portion where the short-circuit is taking place, the transparent conductive oxide is removed at the determined portion by an etchant, followed by deposition of an insulating material on the portion from which the transparent conductive oxide has been removed.
In the method disclosed in U.S. Pat. No. 4,510,674, determination of the portion where short-circuit is taking place is conducted by applying a forward or backward bias voltage to the transparent conductive oxide film and the active region of a solar battery device and detecting the portion at which an electrical current flows. In the method disclosed in U.S. Pat. No. 4,510,675, determination of the portion where short-circuit is taking place is conducted by applying a forward or backward bias voltage to the transparent conductive oxide film and the active region of a solar battery device while applying a light to the device and detecting the portion at which an electrical current flows.
These methods, however, involve the following problems.
(1) A considerably long time is required for the determination of the short-circuit portions particularly when the number of such portions is large.
(2) Application of an etchant tends to cause the non-short-circuit portion of the transparent conductive oxide to be etched, with the result that the performance of the solar battery device is impaired.
U.S. Pat. No. 4,729,970 discloses the following method for discovering any short-circuit path defect occurring in a thin-film type electronic device employing a transparent conductive oxide as an electrode.
An electrical current is supplied to flow between a semiconductor and a counter electrode within a solution containing, as a Lewis acid, salts such as aluminum chloride, zinc chloride, stannic chloride, stannous chloride and titanium tetrachloride. As a result, the transparent conductive oxide is oxidized or reduced so as to change the chemical stoichiometric ratio of the transparent conductive oxide, thereby increasing the resistivity of this film.
This proposed method was tested by employing an aqueous solution of zinc chloride, stannic chloride or stannous chloride, while using the substrate and the counter electrode of the solar battery as a cathode and an anode. The result, however, was quite contrary to what had been expected. Namely, the short-circuit portion was not removed; rather, the leak current due to a short-circuit was undesirably increased as a result of the precipitation of metal-salt formers such as Zn and Sn on the short-circuit portion.
A method for improving efficiency of amorphous silicon (a-Si) solar battery device through electrochemical processing was proposed in 29p-z-5 of Pre-print of Conference of Applied Physics, autumn, 1986. According to this method, a pin-holed portion causing a leak of electrical current in a solar battery of glass/ITO/pin a-Si:H/Al type is repaired by a process including the steps of dissolving the Al of the device in an acid, immersing the sample of the solar battery to be repaired in a dilute sulfuric acid together with platinum, applying a negative potential to the a-Si:H side of the battery so as to electrochemically dissolve ITO (Indium Tin Oxide), and evaporation-depositing Al again so as to complete the battery.
This proposed method for repairing a short-circuit portion, however, can be applied only to solar battery devices of the type in which an ITO layer, an a-Si:H layer as the pin layer and Al layer are sequentially formed on a glass substrate. It cannot be efficiently applied to a photoelectric conversion device of the type having a photoelectric converting semiconductor layer and a transparent conductive layer sequentially formed on a conductive substrate, because of the difficulty encountered in applying a negative potential to the semiconductor layer through the transparent conductive layer which covers the semiconductor layer.
It would be possible to apply a voltage to the semiconductor layer by locally removing the transparent conductive layer by etching. Such a method, however, can produce an effect only in the region around the portion where the voltage is applied. This is particularly so when the photoelectric conversion device has a large area, because the semiconductor layer itself inherently has a large electrical resistivity.
As will be understood from the foregoing description, although various methods have been proposed for eliminating any shunting portion where electric current shunts and short-circuit portions exist in photoelectric conversion devices, all these methods are still unsatisfactory. Namely, the methods relying upon an energy beam require a preparatory work in locating the short-circuit portion and, hence, lower the production efficiency due to impractical long processing time, particularly when short-circuits are taking place in a great number of portions. Practical conditions have not as yet been established for the electrochemical process. Under these circumstances, there is a strong demand for a method for eliminating short-circuit portion which can be applied even to photoelectric conversion devices having a large area and which enables elimination of a short-circuit portion with a high degree of efficiency.